Previous work performed in the CRC has confirmed that there is a gender difference in the QT interval with a mean 10ms longer QT interval in women than in men. This difference is not explained by difference in autonomic tone or by menstural cycle variation. The purpose of this study is to examine gender and circadian differences in the dispersion of ventricular repolarization as measured by the QT interval, as measured on the standard 12 lead ECG and by body surface mapping in patients with and without underlying heart disease.